This invention relates to a solidification treatment vessel into which radioactive waste pellets are charged, and then a solidifying material in the state of a liquid or a slurry is poured into the vessel to solidify the radioactive waste pellets. The invention also relates to a method of solidification treatment of such pellets, using the above vessel.
Recently, there has been used a method of treating a radioactive waste in which a concentrated waste liquid, a waste sludge, a used ion exchanger resin or the like is formed into powder by a vertical thin film dryer, and then the powder is formed into pellets of the tablet type or the briquette type by the addition of an appropriate amount of a suitable binder. There has also been used a method in which ash resulting from combustible wastes burnt in an incinerator is suitably sieved, and then is formed into pellets in the same manner as described above. Such pellets are charged into a suitable vessel or container, and then a liquid-state or slurry-state solidifying material of an organic or an inorganic type (e.g. a cement type or a plastic type) is poured into the vessel to uniformly fill the spaces between the pellets, and is solidified in the vessel, thereby forming a stable solid mass.
In the above pouring of the solidifying material, if the specific gravity of the pellets is lower than that of the solidifying material slurry, the pellets tend to float on the surface of the solidifying material slurry at a final stage of the pouring operation. Therefore, without a suitable float prevention means, many pellets float and appear on the surface of the solidifying material.
In one conventional method of preventing such a float phenomenon, after a predetermined amount of pellets are charged into the vessel, an iron frame or the like having a metal net whose mesh is smaller than the size of the pellet is fixed to the upper portion of the vessel, and then the solidifying material slurry is poured into the vessel through the metal net from above this metal net, thereby preventing the floating of the pellets. Then, the vessel is left to stand until the viscosity of the solidifying material increases to such a degree as to restrain the floating of the pellets, and then the iron frame with the metal net is removed from the vessel, and then a required amount of solidifying material is post-filled in the upper space of the vessel.
This conventional method has the following disadvantages:
If the iron frame with the metal net is fixed to the upper portion of the vessel before the pellets are charged into the vessel, a subsequent charging of pellets into the vessel is prevented by the metal net. Therefore, in the above conventional method, the iron frame with the metal net is fixed to the upper portion of the vessel after the pellets are charged into the vessel, and then the solidifying material is poured into the vessel. Thereafter, the iron frame with the metal net is removed for re-use.
However, after the charging of the pellets, the wall of the vessel is stained with dust of the pellets, and the iron frame with the metal net can not be easily attached to the vessel. And besides, a mechanism for the attachment and detachment of the iron frame relative to the vessel is complicated, and an automatic remote control which is desired for preventing the radiation exposure can not be easily achieved.
In addition, when the iron frame with the metal net is to be removed from the vessel after the pouring of the solidifying material, the solidifying material is liable to be scattered over the surroundings, and it is troublesome to handle the thus removed iron frame. To avoid these problems, it can be considered that the iron frame with the metal net is not removed from the vessel even after the pouring of the solidifying material, and is embedded in the solidifying material in the vessel. In this case, also, the iron frame with the metal net must be attached to the upper portion of the vessel after the pellets are charged into the vessel. Therefore, the same problems as described above are also encountered.
In order to charge a Predetermined amount of the pellets into the vessel with preventing the jumping of the pellets out of the vessel and the scattering of the dust, it is preferred that a kind of lid be mounted on the upper portion of the vessel and that the pellets are charged into the vessel via a charge pipe extending through this lid. However, even with this pellet-charging method, it is impossible to charge the pellets, with the iron frame with the metal net (which prevents the floating of the pellets when the solidifying material is poured at a later stage) being beforehand attached to the upper portion of the vessel. Therefore, after the charging of the pellets, the lid is removed from the vessel, and thereafter the iron frame with the metal net must be attached to the upper portion of the vessel. Thus, the same problems as described above are also encountered.